DS1556_10 Datasheet, PDF (5/18 Pages) Maxim Integrated Products – 1M, Nonvolatile, Y2K-Compliant Timekeeping RAM

DS1556 1M, Nonvolatile, Y2K-Compliant Timekeeping RAM
a typical application, the OE signal will be high during a write cycle. However, OE can be active
provided that care is taken with the data bus to avoid bus contention. If OE is low prior to WE
transitioning low, the data bus can become active with read data defined by the address inputs. A low
transition on WE will then disable the outputs t_WEZ after WE goes active.
DATA-RETENTION MODE
The 5V device is fully accessible and data can be written and read only when V_CC is greater than V_PF.
However, when V_CC is below the power-fail point V_PF (point at which write protection occurs) the
internal clock registers and SRAM are blocked from any access. When V_CC falls below the battery switch
point V_SO (battery supply level), device power is switched from the V_CC pin to the internal backup lithium
battery. RTC operation and SRAM data are maintained from the battery until V_CC is returned to nominal
levels.
The 3.3V device is fully accessible and data can be written and read only when V_CC is greater than V_PF.
hen V_CC falls below V_PF, access to the device is inhibited. If V_PF is less than V_SO, the device power is
switched from V_CC to the internal backup lithium battery when V_CC drops below V_PF. If V_PF is greater
than V_SO, the device power is switched from V_CC to the internal backup lithium battery when V_CC drops
below V_SO. RTC operation and SRAM data are maintained from the battery until V_CC is returned to
nominal levels.
All control, data, and address signals must be powered down when V_CC is powered down.
BATTERY LONGEVITY
The DS1556 has a lithium power source that is designed to provide energy for the clock activity, and
clock and RAM data retention when the V_CC supply is not present. The capability of this internal power
supply is sufficient to power the DS1556 continuously for the life of the equipment in which it is
installed. For specification purposes, the life expectancy is 10 years at 25C with the internal clock
oscillator running in the absence of V_CC. Each DS1556 is shipped from Maxim with its lithium energy
source disconnected, guaranteeing full energy capacity. When V_CC is first applied at a level greater than
V_PF, the lithium energy source is enabled for battery backup operation. Actual life expectancy of the
DS1556 will be much longer than 10 years since no internal battery energy is consumed when V_CC is
present.
INTERNAL BATTERY MONITOR
The DS1556 constantly monitors the battery voltage of the internal battery. The Battery Low Flag (BLF)
bit of the Flags Register (B4 of 1FFF0h) is not writable and should always be a 0 when read. If a 1 is ever
present, an exhausted lithium energy source is indicated and both the contents of the RTC and RAM are
questionable.
POWER-ON RESET
A temperature compensated comparator circuit monitors the level of V_CC. When V_CC falls to the power
fail trip point, the RST signal (open drain) is pulled low. When V_CC returns to nominal levels, the RST
signal continues to be pulled low for a period of 40 ms to 200 ms. The power-on reset function is
independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled.
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